This Halloween, as you're surrounded by candy to eat, you might be wondering how sugar substitutes can turn innocent little sweets into disastrous diarrhea triggers. It's complicated, so I asked someone who knows a lot about stomachs and nutrition, my good friend Danielle Zolotnitsky, RD, a registered dietitian who works in an outpatient gastroenterology clinic in Philadelphia.

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The culprit seems to be sugar alcohols, which are sugar substitutes that are often used in candies because they're lower in calories and sweeter than table sugar, Zolotnitsky explains. For example, some common names of sugar alcohols you may see on labels are sorbitol, mannitol, xylitol, isomalt, and hydrogenated starch hydrolysates. The offending gummies, for example, contained Lycasin, a brand name for a sugar substitute called malitol.

Tricky thing is, our bodies can't fully digest sugar alcohols, meaning they can't cross the "walls" of our cells, Zolotnitsky says. As a result, your body sends water to your stomach and intestines to essentially "flush out" the sugar alcohol, she says. "When you have a bunch of sugar alcohols, a bunch of water rushes to your stomach and intestines, you get diarrhea," she says. The leftover sugar alcohol gets fermented in your gut, which causes gas, bloating, and gastrointestinal sounds, Zolotnitsky further details. So, you can see how candies made with these sugar alcohols are a perfect storm for gastrointestinal chaos.

This should serve as a reminder that just because something is "sugar free," doesn't mean that it's necessarily better for you — even if you're someone with diabetes. "Sometimes patients with diabetes or glucose intolerance see the terms 'lower in sugar,' 'sugar free,' or 'no sugar added,' and are eager to try," Zolotnitsky says. "However, these foods still have a carbohydrate amount, and that needs to be considered if you have diabetes." Ultimately, if you are someone with diabetes, the decision to eat sugar-free candies is between you, your doctor, your RD (find a certified diabetes educator to help you with this) — and your GI tract, she says.

For everyone else, it's probably in your best interest to just eat the classic version of your favorite candies this Halloween. "If you don't have diabetes, I really recommend enjoying the real thing — enjoy your dessert, savor it, and revel in the lack of GI distress," Zolotnitsky says. And save the horror scene for whatever scary movie you decide to watch instead.